What kind of harm can koan answers cause to one's practice?

In a recent [meta discussion](https://buddhism.meta.stackexchange.com/questions/271/should-we-have-a-policy-not-to-provide-koan-answers) some people expressed the opinion that giving koan answers may be detrimental to one's practice. In what way, exactly? And if it is so, why did many Buddhists publish koan commentaries for many hundred years?